1. Technical Field
This invention generally relates to test equipment and more specifically relates to equipment for testing properties of materials.
2. Background Art
Electronics have become essential to our modern way of life in the United States. Electronic assemblies are typically made by installing individual components into a printed wiring board (PWB), which are then soldered into place. The PWB makes all the connections between electronic and electrical components using metal paths that are typically etched into the PWB. Modern PWBs have become very sophisticated and complex, and it is not uncommon to have a PWB today that has in excess of twenty layers. Each layer defines conductor paths that connect to one or more other layers. The many different layers allow packing the components very tightly onto a PWB, thereby reducing the overall area of the PWB. This minimization in size of an electronic assembly is essential for many applications where the size of the electronic assembly must be kept very small, as in mobile phones and other hand-held electronic devices.
As the number of layers in printed wiring boards increases, the physical properties of the boards becomes more and more difficult to estimate. Because each layer can include many areas of metal and many areas without metal, the resulting printed wiring board has features that are asymmetrically distributed. Due to the anisotropic nature of these complex printed wiring boards, destructive testing methods have been used to determine their material properties. However, some modem printed wiring boards are sufficiently complex that they are very expensive, with a cost of thousands of dollars for a bare board. Needless to say, spending these high sums of money to perform destructive testing is undesirable. Without a method for economically determining the properties of anisotropic materials such as printed wiring boards in a nondestructive manner, either these properties will be estimated rather than measured, or the cost of determining these properties using destructive testing will remain excessive.
According to the preferred embodiments, an apparatus and method allow accurately determining properties of an anisotropic material, such as a multilayer printed wiring board, in a non-destructive manner using scanning acoustic microscopy. The apparatus and method includes special signal processing that allows determining the time of flight of an input waveform from a transmitter transducer on one side of the printed wiring board to a receiver transducer positioned on the opposite side of the printed wiring board in a position that opposes the transmitter transducer. The special signal processing includes an auto-correlation function and a cepstrum analysis. Once the time of flight is determined, the velocity of the input waveform may be computed. The Young""s modulus and shear modulus for the printed wiring board may then be computed from the velocity of the input waveform. The preferred embodiments allow determining properties of a printed wiring board without the necessity of destructive testing.